How the foetus avoids rejection
As a foetus develops inside the womb, something extraordinary happens. Or rather, doesn’t happen. The mother’s immune system doesn’t attack the foetus. The trick behind this most crucial of hiding acts has eluded medical science for decades, but recent experiments in mice appear to have revealed the secret: “gene silencing” in the cells surrounding the foetus creates an impenetrable barrier to immune cells, stopping them from reaching the foetus.
Immune cells that fight infection are able to pass out of blood vessels and roam tissues searching for foreign cells, so in theory they should find a foetus and attack it. These cells are attracted by cellular distress signals called chemokines, which are released by inflamed tissues. By studying pregnant mice the scientists found that chemokine genes are turned off in the cells of the decidua, the sack that surrounds the foetus. The DNA containing the genes undergoes temporary modification causing it to become bundled tightly together. This “silences” the genes so that chemokines are not produced and immune cells are not recruited when the embryo implants into the womb lining, which causes inflammation.
If this silencing process does not occur properly the mother’s immune system could be attracted to the foetus, leading to complications of pregnancy, including preterm labour, spontaneous abortion and preeclampsia. These experiments in mice can now guide further work looking at human pregnancy to see if similar mechanisms are at play and how complications might be avoided. The findings, which cast light on immune system function, could also have implications for autoimmune diseases, organ transplantation and cancer.